Galvanized steel pipe

Abstract

Ensure corrosion resistance in and around the welded area. [Solution] A plated steel pipe, wherein a coating layer containing a Zn-Al-Mg alloy plating layer exists on a steel substrate on at least the outer surface of the plated steel pipe, and in any cross-section obtained by cutting the plated steel pipe in the diameter direction perpendicular to the longitudinal direction, the coating layer has a region A on the surface of the steel substrate where Fe-Al intermetallic compounds are not continuously present for 20 μm or more, the circumferential length of region A is 4 mm or less, and region A has a metal layer containing one or more metals, either Zn or Al, and the minimum thickness of the coating layer in region A is 3 μm or more.

Description

Technical Field

[0001] The present invention relates to a plated steel pipe.

Background Art

[0002] Steel pipes plated with various zinc-based platings, such as hot-dip galvanized plating, Zn-Al alloy plating, Zn-Al-Mg alloy plating, etc., exhibit excellent corrosion resistance, and thus are widely used as materials for civil engineering and construction members, materials for automotive parts, etc.

[0003] There are several methods for manufacturing such plated steel pipes. One such manufacturing method is to perform bending on a steel sheet plated with various zinc-based platings (so-called zinc-based plated steel sheet) to form it into a cylindrical shape, and weld the portions where the end faces of the steel sheet are abutted against each other.

[0004] In the method of bending and welding plated steel sheets as described above, the welded portion may protrude above the surface of the plated steel sheet. Therefore, a grinding process (also called bead cutting) to remove such protruding portions has been conventionally performed. Furthermore, a polishing process to polish the area around the welded portion has been conventionally performed before this grinding process. More specifically, during the grinding process, the cutting blade used to remove the protruding portion may catch on the plating layer present near the welded portion, generating processing debris from that plating layer. Such processing debris can lead to, for example, a defect in the appearance around the welded portion. Therefore, in the polishing process before grinding, the plating layer in the area where processing debris may be generated is removed in advance. Furthermore, after the grinding process, a removal process to remove the fumes generated during the grinding process has been conventionally performed. Although the protrusion and appearance defects of the welded portion are eliminated by this grinding, polishing, and fume removal process, the plating layer present near the welded portion is also removed, exposing the surface of the steel sheet. Since exposed areas on the surface of steel plates have lower corrosion resistance than areas where the plating layer is properly present, it is common practice to repair them by thermal spraying metallic aluminum (Al) or metallic zinc (Zn) onto the welded areas and exposed areas to ensure the corrosion resistance of the plated steel pipe.

[0005] For example, Patent Documents 1 to 4 below describe a method of repairing a welded joint by performing grinding, polishing, or fume removal on the protruding portion to a predetermined width, and then thermal spraying metallic Al or metallic Zinc onto the surface treatment area where the steel plate surface is exposed. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2012-107324 [Patent Document 2] Japanese Patent Publication No. 2007-191800 [Patent Document 3] Japanese Patent Publication No. 2021-123781 [Patent Document 4] Japanese Patent Publication No. 2016-084531 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, in the layers formed by thermal spraying metals such as Al or Zinc onto the welded areas and exposed areas treated as described above, there may be areas with insufficient thickness. This is thought to be because the exposed areas become too large in the polishing and fume removal processes described above, resulting in insufficient supply of thermal spray metal to the entire surface treatment area. Corrosion of the steel plate surface may occur in these areas with insufficient thickness. Therefore, there is room for improvement in ensuring the corrosion resistance of the welded areas and their vicinity in steel pipes. In particular, in the case of steel pipes using Zn-Al-Mg plated steel sheets with high corrosion resistance developed in recent years, the corrosion resistance of areas other than the welds has improved significantly, so poor corrosion resistance of the welds becomes the rate-limiting factor in the lifespan.

[0008] The object of the present invention is to provide a steel pipe that can ensure corrosion resistance in and around the welded joint. [Means for solving the problem]

[0009] The gist of this invention is as follows: (1) Galvanized steel pipe, At least on the outer surface of the plated steel pipe, a coating layer containing a Zn-Al-Mg alloy plating layer exists on the steel substrate. In any cross-section obtained by cutting the aforementioned plated steel pipe in the radial direction perpendicular to the longitudinal direction, The aforementioned coating layer is The surface of the steel substrate has a region A where Fe-Al intermetallic compounds are not continuously present for 20 μm or more. The circumferential length of region A is 4 mm or less. In the region A, there is a metal layer containing one or more metals, namely Zn and Al. The minimum thickness of the coating layer in region A is 3 μm or more. Galvanized steel pipe. (2) The coating layer is Region A has a region B that extends 1 mm in the circumferential direction from the end of region A in the opposite direction to region A, The plated steel pipe according to (1) above, wherein in region B, it has the Zn-Al-Mg alloy plating layer, and on the Zn-Al-Mg alloy plating layer, it has a metal layer containing one or more metals, either Zn or Al. (3) The coating layer is Region C is a circumferential region of 1 mm at any position within a range of 10 to 15 mm in the circumferential direction from the end of region A, in the opposite direction from region A. The average thickness of the Zn-Al-Mg alloy plating layer in region B is 50% or more of the average thickness of the coating layer in region C. The plated steel pipe according to (2) above, wherein the minimum thickness of the coating layer in region B is 3 μm or more. (4) The plated steel pipe according to any one of the above items (1) to (3), wherein the region A is a region that covers the welded portion of the base steel pipe constituting the plated steel pipe. (5) The Zn-Al-Mg alloy plating layer is, by mass%, Al: more than 1.0% and less than 30.0%, Mg: greater than 1.0% and less than or equal to 15.0%, and The remainder: A plated steel pipe according to any one of the above items (1) to (3), having a chemical composition consisting of Zn and impurities. (6) The Zn-Al-Mg alloy plating layer is, by mass%, Al: more than 1.0% and less than 30.0%, Mg: More than 1.0% and less than 15.0% A plated steel pipe according to (1) or (2) above, which contains, and further contains one or more elements selected from the group consisting of element groups A to E below, with the remainder being Zn and impurities, having a chemical composition. [Element group A]:Fe:5.00% or less [Element Group B]: One or more elements selected from the group consisting of Ti: 0.25% or less, Ni: 1.00% or less, and Co: 0.25% or less. [Element Group C]: Sn: 0.70% or less [Element Group D]: Ca: 0.60% or less [Element Group E]: B: 0.50% or less (7) The Zn-Al-Mg alloy plating layer has, in mass %, Al: more than 1.0% and 30.0% or less, Mg: more than 1.0% and 15.0% or less contains, and further contains one or more selected from the group consisting of the following Element Groups A to Element Groups E, and the balance consists of Zn and impurities, and has a chemical composition, the plated steel pipe according to (3) above. [Element Group A]: Fe: 5.00% or less [Element Group B]: one or more selected from the group consisting of Ti: 0.25% or less, Ni: 1.00% or less, Co: 0.25% or less [Element Group C]: Sn: 0.70% or less [Element Group D]: Ca: 0.60% or less [Element Group E]: B: 0.50% or less (8) The Zn-Al-Mg alloy plating layer possessed by at least one of the Region B and the Region C has a chemical composition having the Element Group A, and the plated steel pipe according to (7) above. (9) The Zn-Al-Mg alloy plating layer possessed by at least one of the Region B and the Region C has a chemical composition having the Element Group B, and the plated steel pipe according to (7) above. (10) The Zn-Al-Mg alloy plating layer possessed by at least one of the Region B and the Region C has a chemical composition having the Element Group C, and the plated steel pipe according to (7) above. (11) The Zn-Al-Mg alloy plating layer possessed by at least one of the Region B and the Region C has a chemical composition having the Element Group D, and the plated steel pipe according to (7) above. (12) The Zn-Al-Mg alloy plating layer possessed by at least one of the Region B and the Region C has a chemical composition having the Element Group E, and the plated steel pipe according to (7) above. (13) A plated steel pipe, At least on the outer surface of the plated steel pipe, a coating layer containing a Zn-Al-Mg alloy plating layer exists on the steel substrate. In any cross-section obtained by cutting the aforementioned plated steel pipe in the radial direction perpendicular to the longitudinal direction, The coating layer has a minimum thickness of 3 μm or more in a region extending 5 mm on each side of the circumferential direction from the center of the welded area. Galvanized steel pipe. [Effects of the Invention]

[0010] According to the present invention, it is possible to ensure corrosion resistance of the welded area and its vicinity. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic cross-sectional view showing a cross-section obtained by cutting a steel pipe according to one embodiment in the radial direction perpendicular to the longitudinal direction. [Figure 2] This is a schematic enlarged cross-sectional view showing a cross-section near the weld in a steel pipe according to one embodiment. [Figure 3] This is an explanatory diagram illustrating a second region having a second coating layer in a steel pipe according to one embodiment. [Figure 4] This is a schematic enlarged cross-sectional view showing a cross-section near the weld in a steel pipe according to one embodiment. [Modes for carrying out the invention]

[0012] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant descriptions will be omitted.

[0013] (Regarding steel pipes) In the following, a steel pipe 1 according to an embodiment of the present invention will be described in detail with reference to Figures 1 to 3.

[0014] <Regarding the overall structure of steel pipes> Figure 1 is a schematic cross-sectional view showing a cross-section obtained by cutting the steel pipe 1 according to this embodiment in the radial direction perpendicular to the longitudinal direction. Figure 2 is an explanatory diagram for explaining the coating layer 20 in the steel pipe according to this embodiment, and is an enlarged view of the vicinity of the welded portion 13 shown in Figure 1.

[0015] For convenience, the following explanation will refer to the XYZ coordinate system shown in Figure 1, and will use this coordinate system as needed. In this coordinate system, the Z-axis direction is parallel to the longitudinal direction (pipe axis direction) of the steel pipe 1 according to this embodiment, and the X-axis and Y-axis directions are perpendicular to the Z-axis direction, respectively.

[0016] The steel pipe 1 according to this embodiment is suitably used as a material for manufacturing various components such as solar power generation panel mounting frames and other structures, automobile parts, various building materials, pillars, signs, traffic lights, guardrails, etc. As schematically shown in Figure 1, the steel pipe 1 according to this embodiment has a raw steel pipe 10 and a coating layer 20 provided on the outer surface of the raw steel pipe 10.

[0017] The coating layer 20 of the steel pipe 1 according to this embodiment is a layer provided to ensure the corrosion resistance of the steel pipe 1 according to this embodiment, and has a specific configuration as detailed below. By having such a coating layer 20, the steel pipe 1 according to this embodiment exhibits the desired excellent corrosion resistance.

[0018] The steel pipe 1 according to this embodiment is manufactured by bending a plated steel sheet, which has a Zn-Al-Mg alloy plating layer (hereinafter sometimes simply referred to as the "plating layer") formed on a base steel sheet, to form a cylindrical shape, welding the parts where the ends of the steel sheets butt together, and then performing post-processing such as grinding or repair on the protruding parts resulting from the welding.

[0019] Here, the size (outer diameter) of the steel pipe 1 according to this embodiment is assumed to be in the range of 10 to 160 mm. Preferably, the size (outer diameter) of the steel pipe 1 is in the range of 60 to 145 mm. The shape of the steel pipe is not limited to a tubular shape; it may also be a square pipe, a shaped pipe, or an expanded pipe with a partially changed outer diameter, and its shape is not restricted. Furthermore, the steel pipe is not limited to a straight pipe with a straight longitudinal axis, but may also be a curved pipe with a bent shape.

[0020] Further details regarding the base steel pipe 10 and the coating layer 20 described above will be explained in more detail below.

[0021] <Regarding the unpainted steel pipe 10> As schematically shown in Figure 1, the base material of the steel pipe 1 according to this embodiment, the raw steel pipe 10, is composed of a steel base 11 and a welded joint 13.

[0022] Regarding steel substrate 11: The steel base material 11 in the base steel pipe 10 corresponds to the base steel sheet portion of the Zn-Al-Mg alloy plated steel sheet, which is the material of the steel pipe 1 according to this embodiment. The steel base material 11 according to this embodiment is not particularly limited, and various types of steel materials can be used depending on the mechanical strength (e.g., tensile strength) required for the steel pipe 1. Examples of such steel materials include various types of Al-killed steel, ultra-low carbon steel containing Ti, Nb, etc., high-strength steel further containing reinforcing elements such as P, Si, Mn in ultra-low carbon steel, and various other steel materials containing various components (Cr, N, Cu, B, Ni, Mg, Ca, V, Co, Zn, As, Y, Zr, Mo, Sn, Sb, Ta, W, Pb, Bi, REM, etc.).

[0023] Furthermore, the surface of the steel substrate 11 is provided with an intermetallic compound layer 12, which may be formed when manufacturing a Zn-Al-Mg alloy plated steel sheet, the material for the steel pipe 1 according to this embodiment. The components constituting the intermetallic compound layer 12 vary depending on the chemical composition of the steel substrate 11 and the chemical composition of the Zn-Al-Mg alloy plated layer provided on the steel substrate 11. In the following description, the term "surface of the steel substrate 11" shall be interpreted as including the surface of the intermetallic compound layer 12.

[0024] As will be described later, the intermetallic compound layer 12 does not necessarily exist continuously on the steel substrate where the plating layer is present around the entire circumference of the steel pipe 1 when observed in cross-section, but may exist intermittently with intervals of less than 20 μm. Also, as will be described later, the intermetallic compound layer 12 may not be present in the grinding and polishing portion 14 on the surface, which will be described later.

[0025] In this embodiment, the intermetallic compound layer 12 is substantially composed of an Fe-Al intermetallic compound. The Fe-Al intermetallic compound is not limited to this, but can be formed when forming the plating layer on the steel substrate 11 when the plating alloy used as the material for forming the Zn-Al-Mg alloy plating layer does not contain Si. The region of the intermetallic compound layer 12 can be identified as a region containing Fe and Al in elemental mapping analysis by SEM-EDX, which will be described later.

[0026] Furthermore, the thickness of the steel substrate 11 is not particularly limited and can be set appropriately according to the mechanical strength and other requirements for the steel pipe 1 according to this embodiment.

[0027] Regarding the welded section 13: The welded portion 13 in the raw steel pipe 10 according to this embodiment is formed by welding the butt joint portion where the end faces of the Zn-Al-Mg alloy plated steel sheet, which is the material of the steel pipe 1 according to this embodiment, are joined together after bending the steel sheet into a cylindrical shape. The welded portion 13 is the part where the butt joint portion is melted and joined, and is also called the bond portion. The welding method used when welding the butt joint portion is not particularly limited, and various welding methods such as high-frequency welding, other electric resistance welding, laser welding, and arc welding can be used.

[0028] The components of the welded joint 13 vary depending on the chemical composition of the steel substrate 11, the chemical composition of the Zn-Al-Mg alloy plating layer provided on the steel substrate 11, and the chemical composition of the welding wire used as needed during welding. For example, the components of the welded joint 13 are generally mainly oxides of easily oxidized elements among the various elements that make up the steel substrate 11, the Zn-Al-Mg alloy plating layer, and the welding wire.

[0029] Here, in the cross-section of the steel pipe 1 according to this embodiment (a cross-section obtained by cutting the steel pipe 1 radially so as to be perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1)), the length of the heat-affected zone including the welded portion 13 along the circumferential direction of the steel pipe 1 is not specifically defined, but is generally 60 mm or less, regardless of the outer diameter of the steel pipe 1. Furthermore, the circumferential length of the welded portion 13 as a bond is preferably 4 mm or less.

[0030] Furthermore, when identifying the portion corresponding to the welded joint 13 in the cross-section of the steel pipe 1 of interest, it can be easily visualized by etching using an etching solution. For example, as an etching solution, it is possible to use Nital (composition: 95% ethanol, 5% sulfuric acid) or an etching solution prepared by mixing 60g of sodium dodecylbenzenesulfonate, 36g of picric acid, 60cc of ethanol, and 60cc of household detergent solution (for example, a common type such as dish soap) with 2400cc of water.

[0031] <Regarding the grinding and polishing section 14> The grinding and polishing portion 14 is a portion formed by a grinding process during the manufacturing process of the steel pipe 1, which removes protruding portions that protrude at least beyond the surface of the plated steel sheet. As will be described later, in the manufacturing process of the steel pipe 1 according to this embodiment, polishing treatment (fume removal treatment) is not performed before or after grinding. On the other hand, in the manufacturing process of conventional steel pipes, polishing treatment may be performed before and after grinding. In such steel pipes, the grinding and polishing portion 14 may include the portion that has undergone such polishing treatment. Note that the grinding and polishing portion 14 is defined for the convenience of explaining the structure of the steel pipe 1 according to this embodiment.

[0032] <Regarding the coating layer 20> The coating layer 20 of the steel pipe 1 according to this embodiment is a layer located on the outer surface of the raw steel pipe 10 as described above, and is provided to ensure the corrosion resistance of the steel pipe 1. The inner surface of the steel pipe may have the same coating layer as the coating layer 20 on the outer surface, or it may have a different coating layer. Even a raw steel pipe 10 without a coating layer 20 is within the scope of the present invention.

[0033] As schematically shown in Figures 1 and 2, the coating layer 20 is composed of three regions: a first region 21 that occupies most of the outer surface of the steel pipe 1, a second region 23 that is provided to at least cover the welded portion 13 of the base steel pipe 10, and a third region 25 located between the first region 21 and the second region 23.

[0034] The first region 21 of the coating layer 20 is the region in which the Zn-Al-Mg alloy plating layer that was present on the Zn-Al-Mg alloy plated steel sheet, which is the material of the steel pipe 1 according to this embodiment, remains even after it has become a steel pipe. In this first region 21, as will be described in detail below, there is a first coating layer 211 that corresponds to the Zn-Al-Mg alloy plating layer of the plated steel sheet that was the material.

[0035] Furthermore, the second region 23 of the coating layer 20 is a region formed by a repair treatment of the grinding and polishing portion 14, which is the part where a protruding portion generated during the manufacture of the steel pipe 1 has been ground. The second region 23 corresponds to region A of this disclosure. The circumferential length of the second region 23 is equal to the circumferential length of the grinding and polishing portion 14. In the grinding and polishing portion 14, the Zn-Al-Mg alloy plating layer and the intermetallic compound layer 12 have been removed by the grinding. In the second region 23, there is a second coating layer 231 formed by a repair treatment, for example using thermal spraying, as a layer to ensure the corrosion resistance of the grinding and polishing portion 14, including the welded portion 13.

[0036] Furthermore, the third region 25 of the coating layer 20 is a region located between the first region 21 and the second region 23, in which a part of the Zn-Al-Mg alloy plating layer as the first coating layer 211 undergoes changes such as (a) thinning, (b) slight changes in the plating composition, (c) the inclusion of oxides, or (d) disappearance due to the effects of heat input during welding. The second coating layer 231 is present on the first coating layer 211 where the above changes (a) to (c) have occurred in the third region 25, or on the steel substrate 11 or intermetallic compound layer 12 that has been exposed due to the disappearance of the first coating layer 211 as described in (d) above.

[0037] The first to third regions 21 to 25 constituting the coating layer 20 will be described in detail below. Note that in Figure 1, the illustration of the coating layer that may exist on the inner surface of the steel pipe 1 according to this embodiment is omitted. However, it is preferable that the inner surface of the steel pipe 1 according to this embodiment has a first coating layer derived from a Zn-Al-Mg alloy plating layer, as detailed below, on the surface of the portion corresponding to the steel substrate 11.

[0038] <Regarding Area 1, Item 21> As described above, the first region 21 in this embodiment is a region in which the Zn-Al-Mg alloy plating layer, which was present in the Zn-Al-Mg alloy plated steel sheet that is the material of the steel pipe 1 in this embodiment, remains even after it has become a steel pipe. Below, this first region 21 will be described in detail with reference to Figures 1 and 2. Figure 2 is an enlarged view of the area near the boundary between the first region 21 and the third region 25 in Figure 1, and is an explanatory diagram for describing the first region 21 having the first coating layer 211 in the steel pipe according to this embodiment.

[0039] As shown in Figure 2, a first coating layer 211 exists in the first region 21. The first coating layer 211 contains Zn, Al, and Mg. The thickness of the first coating layer 211 is not particularly defined, but in one embodiment, there is a region with a thickness of 8.0 μm or more that extends continuously for 1.0 mm or more along the circumferential direction of the steel pipe 1. Here, the "circumferential direction" in this embodiment is the direction shown in Figure 2, which is perpendicular to the longitudinal direction of the steel pipe (the Z-axis direction in Figure 1, i.e., the imaginary line passing through the center of the cross-section of the steel pipe) and along the surface of the steel pipe.

[0040] Regarding the method for identifying Area 1, Section 21: In any cross-section of the steel pipe 1 according to this embodiment, cut perpendicular to the diameter direction (i.e., perpendicular to the longitudinal direction of the pipe), as shown in Figure 1, the portion corresponding to the first region 21 can be easily identified by observing the cross-section of the steel pipe 1 of interest with a SEM equipped with an energy dispersive X-ray detector (EDX) (hereinafter abbreviated as "SEM-EDX") and performing elemental mapping analysis. In such elemental mapping analysis, it is sufficient to focus on at least the elements Zn, Al, and Mg, or all elements may be considered.

[0041] More specifically, in the elemental mapping analysis using SEM-EDX as described above, the region containing the Zn phase, Zn / Al phase, and Zn / Al / MgZn2 ternary eutectic phase can be identified as the region corresponding to the first region 21.

[0042] ≪Regarding the chemical composition of the first coating layer 211≫ The preferred chemical composition of the first coating layer 211 according to this embodiment is, in one embodiment, as described above, a chemical composition in which, by mass%, Al: more than 1.0% and 30.0% or less, Mg: more than 1.0% and 15.0% or less, with the remainder being Zn and impurities.

[0043] Furthermore, according to another embodiment, the preferred chemical composition of the first coating layer 211 in this embodiment is, by mass%, Al: greater than 1.0% and 30.0% or less, Mg: greater than 1.0% and 15.0% or less, and further, it contains one or more elements selected from the group consisting of element group A, element group B, element group C, element group D, and element group E, with the remainder being Zn and impurities.

[0044] [Element group A]:Fe:5.00% or less [Element Group B]: One or more elements selected from the group consisting of Ti: 0.25% or less, Ni: 1.00% or less, and Co: 0.25% or less. [Element group C]: Sn: 0.70% or less [Element group D]:Ca: 0.60% or less [Element group E]:B:0.50% or less

[0045] [Al: more than 1.0 mass% and 30.0 mass% or less] Al is an element necessary to constitute the main metal structure (Zn-Al-Mg metal structure) in the first coating layer 211 according to this embodiment, and is included in a predetermined amount or more to ensure corrosion resistance as a plated steel pipe. If the Al content in the first coating layer 211 is 1.0 mass% or less, desirable corrosion resistance cannot be guaranteed. Therefore, in the first coating layer 211 according to this embodiment, the Al content is preferably greater than 1.0 mass%. By having an Al content within the above range, it is possible to ensure desirable corrosion resistance as a plated steel pipe.

[0046] On the other hand, if the Al content in the first coating layer 211 exceeds 30.0% by mass, the Al phase, which functions as a cathode when placed in a corrosive environment, increases excessively, making the corrosion of the steel pipe 1 more likely to progress, and thus desirable corrosion resistance cannot be guaranteed. For this reason, in the first coating layer 211 according to this embodiment, the Al content is 30.0% by mass or less. More preferably, the Al content is 23.0% or less.

[0047] [Mg: more than 1.0 mass% and 15.0 mass% or less] Mg is an element necessary to constitute the main metal structure (Zn-Al-Mg metal structure) in the first coating layer 211 according to this embodiment, and is included in a predetermined amount or more to ensure corrosion resistance as a plated steel pipe. If the Mg content in the first coating layer 211 is 1.0% by mass or less, desirable corrosion resistance cannot be guaranteed. Therefore, in the first coating layer 211 according to this embodiment, the Mg content is greater than 1.0% by mass. The Mg content is preferably 5.5% by mass or more. By having an Mg content within the above range, it is possible to guarantee desirable corrosion resistance as a plated steel pipe.

[0048] On the other hand, if the Mg content in the first coating layer 211 exceeds 15.0% by mass, the anode dissolution of the first coating layer 211 is more likely to progress when placed in a corrosive environment, which may make it impossible to guarantee the corrosion resistance of the plated steel pipe. Therefore, in the first coating layer 211 according to this embodiment, the Mg content is 15.0% by mass or less. Preferably, the Mg content is 10.0% by mass or less. By keeping the Mg content within the above range, it is possible to guarantee desirable corrosion resistance of the plated steel pipe.

[0049] In the first coating layer 211 according to this embodiment, the remainder of the Al and Mg consists of Zn and impurities. Zn is an element necessary for constituting the main metal structure (Zn-Al-Mg metal structure) in the first coating layer 211 according to this embodiment, and is an important element for improving the corrosion resistance of the plated steel pipe. Furthermore, by containing Al and Mg within the above ranges, and also containing Zn, the first coating layer 211 can ensure the desirable corrosion resistance required for plated steel pipes.

[0050] Next, in a preferred configuration of the first coating layer 211 according to another embodiment of this model, the element groups A to E that the chemical composition of the first coating layer 211 may have will be described in detail.

[0051] In addition, in the first coating layer 211 according to another embodiment of this embodiment, if at least one of the elements belonging to element groups A to E below is included, it is preferable that at least one of the elements belonging to element groups A to E below is included within the following content range.

[0052] ◇Element group A In another embodiment of the first coating layer 211 according to this embodiment, the element group A that the first coating layer 211 may contain will be described. The elements of element group A shown below are elements that may be contained in the first coating layer 211 in place of a portion of the remaining Zn. [Element group A]:Fe:5.00% or less

[0053] [Fe:0~5.00% by mass] The first coating layer 211 may contain elements that make up the steel substrate 11, which is the base material. In particular, in the hot-dip galvanizing method, the steel substrate 11 dissolves into the molten plating bath, and if production continues continuously, the Fe concentration in the bath increases, which can increase the Fe concentration in the first coating layer 211. Due to such elemental contamination, the first coating layer 211 often contains a predetermined amount of Fe, and although the content may be 0% by mass, it is preferable to add Fe to the bath to suppress excessive dissolution of the steel substrate during plating, and therefore, the Fe content in the plating layer is preferably 0.1% or more.

[0054] Furthermore, within a range that does not impair the effects of the present invention, Fe may be intentionally added to the plating bath used when manufacturing the first coating layer 211. However, if the Fe content in the plating bath increases, high-melting-point intermetallic compounds of Fe and Al will form in the plating bath. In this case, such high-melting-point intermetallic compounds tend to adhere to the first coating layer 211 as dross, significantly degrading the appearance quality, which is undesirable. From this viewpoint, the Fe content in the plating bath is adjusted. The Fe content in the first coating layer 211 is preferably 5.00% by mass or less. The Fe content in the first coating layer 211 is more preferably 3.00% by mass or less, and even more preferably 2.00% by mass or less, 1.00% by mass or less, or 0.50% by mass or less.

[0055] ◇Element group B In another embodiment of the first coating layer 211 according to this embodiment, the element group B that the first coating layer 211 may contain will be described. At least one of the elements of element group B shown below may be contained in the first coating layer 211 in place of a portion of the remainder of Zn. [Element Group B]: One or more elements selected from the group consisting of Ti: 0.25% or less, Ni: 1.00% or less, and Co: 0.25% or less.

[0056] [Ti:0~0.25% by mass] [Ni:0~1.00% by mass] [Co:0~0.25% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that it does not contain Ti, Ni, or Co, so the lower limit of the content of these elements is 0 mass%. On the other hand, if at least one of Ti, Ni, or Co is contained in the first coating layer 211, when such plated steel sheet is welded, these elements are incorporated into the Fe-Al metal structure generated by welding, making it possible to further improve the corrosion resistance of the welded part 13. This effect of improving the corrosion resistance of the welded part is manifested when the content of at least one of Ti, Ni, or Co in the first coating layer 211 is 0.05 mass% or more. Therefore, when at least one of Ti, Ni, or Co is contained in the first coating layer 211, it is preferable that the content of each of these elements be independently 0.05 mass% or more.

[0057] On the other hand, when forming a first coating layer 211 in which the content of either Ti or Co exceeds 0.25 mass%, or the content of Ni exceeds 1.00 mass%, these elements may form various intermetallic compounds in the plating bath used to form the first coating layer 211, leading to an increase in the viscosity of the plating bath and potentially making it impossible to produce a plated steel sheet with good plating properties. Therefore, it is preferable that the content of Ti, Ni, and Co in the first coating layer 211 be independently 0.25 mass% or less for Ti and Co, and 1.00 mass% or less for Ni. Preferably, the content of Ti, Ni, and Co is independently 0.20 mass% or less for Ti and Co, and 0.85 mass% or less for Ni.

[0058] ◇Element group C In another embodiment of the first coating layer 211 according to this embodiment, the element group C that the first coating layer 211 may contain will be described. The elements of element group C shown below are elements that may be contained in the first coating layer 211 in place of a portion of the remaining Zn. [Element group C]: Sn: 0.70% or less

[0059] [Sn:0~0.70% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that it does not contain Sn, so the lower limit of the Sn content is 0 mass%. On the other hand, Sn is an element that improves the dissolution rate of plating when a plating layer containing Zn, Al, and Mg is placed in a corrosive environment. By including Sn, when the steel substrate is exposed at cut ends or damaged areas, the plating components are dissolved early, protecting the exposed steel substrate early and suppressing corrosion of the steel substrate, thereby improving the corrosion resistance of the entire member. This effect of improving corrosion resistance is manifested when the Sn content in the first coating layer 211 is 0.005 mass% or more. Therefore, when Sn is included in the first coating layer 211, it is preferable that the Sn content be 0.005 mass% or more.

[0060] On the other hand, excessive Sn content can lead to excessive leaching of the plating layer, potentially reducing the corrosion resistance of the component. This phenomenon becomes particularly noticeable when the Sn content exceeds 0.70% by mass. Therefore, it is preferable that the Sn content be 0.70% by mass or less. It is even more preferable that the Sn content be 0.05% by mass or less.

[0061] ◇Element group D Next, in another embodiment of the first coating layer 211 according to this embodiment, the element group D that the plating layer 103 may contain will be described. The elements of element group D shown below are elements that may be contained in the first coating layer 211 in place of a portion of the remaining Zn. [Element group D]:Ca: 0.60% or less

[0062] [Ca:0~0.60% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that it does not contain Ca, so the lower limit of its content is 0 mass%. On the other hand, if Ca is included in the plating bath for manufacturing the first coating layer 211, it is possible to reduce the dross generated as the Mg concentration increases during plating operations, thereby improving plating operability.

[0063] Furthermore, when Ca is included in the first coating layer 211, it forms intermetallic compounds with Al and Zn. When Ca is included in the first coating layer 211, the Ca content in the first coating layer 211 is more preferably 0.05% by mass or more.

[0064] On the other hand, if the Ca content in the first coating layer 211 exceeds 0.60% by mass, the corrosion resistance of the plated steel pipe may decrease. From this viewpoint, it is preferable that the Ca content in the first coating layer 211 be 0.60% by mass or less. More preferably, the Ca content in the first coating layer 211 is 0.40% by mass or less.

[0065] ◇Element group E In another embodiment of the zinc-based plating layer according to this embodiment, the element group E that the zinc-based plating layer may contain will be described. The elements of element group E shown below are elements that may be contained in the zinc-based plating layer in place of a portion of the remaining Zn. [Element group E]:B:0.50% or less

[0066] [B:0~0.50% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that B is not contained, so the lower limit of its content is 0 mass%. On the other hand, when B is contained in the first coating layer 211, it has the effect of further suppressing LME. This is presumed to be because when B is contained in the first coating layer 211, it combines with at least one of Zn, Al, Mg, and Ca to form various intermetallic compounds. Furthermore, it is thought that the presence of B in the first coating layer 211 causes B to diffuse from the first coating layer 211 to the steel substrate 11, and that this has the effect of further suppressing LME of the steel substrate 11 through grain boundary strengthening. Moreover, it is presumed that the various intermetallic compounds formed with respect to B have extremely high melting points and therefore also act to suppress Zn evaporation during welding. These improvement effects are achieved when B is contained at a concentration of 0.03 mass% or more. Therefore, when B is included, it is preferable that the B content be 0.03 mass% or more.

[0067] On the other hand, if an excessive amount of B is added to the plating bath in order to include B in the first coating layer 211, it can cause a rapid increase in the plating melting point, leading to a decrease in plating operability and potentially making it impossible to produce plated steel sheets with excellent plating properties. This decrease in plating operability becomes particularly noticeable when the B content exceeds 0.50% by mass, so it is preferable that the B content be 0.50% by mass or less. More preferably, the B content is 0.30% by mass or less.

[0068] [Method for measuring chemical components] The chemical composition of the first coating layer 211 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry). When analyzing chemical components down to 0.1 mass%, ICP-AES should be used, while for analyzing trace amounts of chemical components less than 0.1 mass%, ICP-MS should be used. In this embodiment, a sample measuring 30 mm x 30 mm in plan view is cut from a portion of the steel pipe 1 sufficiently far from the end of the welded portion 13 (for example, 15 mm or more away). The collected sample is immersed in a 10% HCl aqueous solution with an inhibitor for about 1 minute to peel off the portion of the first coating layer 211, and a solution is prepared in which this first coating layer 211 is dissolved. The obtained solution can be analyzed by ICP-AES or ICP-MS to obtain the overall average chemical composition of the first coating layer 211.

[0069] Regarding the amount of the first coating layer 211 attached: The amount of the first coating layer 211 that adheres (the amount that adheres to the outer surface of the steel pipe 1) as described above is, for example, 50.0 to 300.0 g / m². 2 It is preferable that the concentration be around 70.0 to 220.0 g / m². 2It is more preferable that the amount of adhesion of the first coating layer 211 falls within the above range. As a result, the steel pipe 1 according to this embodiment can exhibit sufficient corrosion resistance. Furthermore, as a result of the amount of adhesion of the first coating layer 211 falling within the above range, the average thickness of the first coating layer 211 will be approximately 8 to 50 μm. The "amount of adhesion" is measured, for example, by cutting out an arbitrary portion of the first coating layer of any area in accordance with JIS K3323 Annex E and measuring it by gravimetric method.

[0070] The amount of the first coating layer 211 can be measured as follows. First, a sample measuring 30 mm x 30 mm in plan view is cut from a point sufficiently far from the end of the welded joint 13 (for example, a point 15 mm or more away), and the mass of the sample is measured. Then, protective tape, such as a tape-like seal, is applied to the surface of the sample corresponding to the inner surface of the steel pipe 1 to prevent the first coating layer 211 that may be present on this surface from dissolving in the next step. The type of protective tape is not limited as long as it adheres closely to the inner surface of the steel pipe 1 and does not allow the 10% HCl aqueous solution to penetrate. After that, the sample is immersed in the 10% HCl aqueous solution with the inhibitor added, and the first coating layer 211 on the side without the protective tape is pickled and removed, and the mass of the sample after pickling is measured. The amount of the first coating layer 211 can be determined from the change in mass of the sample before and after pickling.

[0071] ≪Regarding the circumferential length of the first region 21≫ Let us focus on an arbitrary cross-section obtained by cutting the steel pipe 1 according to this embodiment in the radial direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1). In such a cross-section, the first region 21 composed of the first coating layer 211 having the above-described chemical composition preferably has a region where the thickness of the first coating layer 211 is 15.0 μm or more, and this region extends continuously for 1.0 mm or more along the circumferential direction of the steel pipe 1.

[0072] Here, if the circumferential length of the region where the thickness of the first coating layer 211 is 15.0 μm or more is continuously less than 1.0 mm, the coating state of the first coating layer 211 on the steel pipe 1 is insufficient, and corrosion resistance as a plated steel pipe cannot be guaranteed. The longer the circumferential length of the region where the thickness of the first coating layer 211 is 15.0 μm or more, the better, and there is no particular upper limit.

[0073] The circumferential length of the first coating layer 211, as described above, is determined by observing an arbitrary cross-section of interest using a scanning electron microscope (SEM) (for example, a JSM-7000F manufactured by JEOL Ltd.) at a magnification of approximately 500x, and measuring the length at the interface between the first coating layer 211 and the steel substrate 11 using the length measuring function implemented in the SEM. In such measurement, it is sufficient that there is a continuous region of 1.0 mm or more in which the thickness of the first coating layer 211 is 15.0 μm or more.

[0074] <Regarding Area 2, Item 23> Next, with reference to Figures 1 to 3, the second region 23 of the coating layer 20 according to this embodiment will be described in detail.

[0075] The second region 23 in this embodiment is a region formed by a repair treatment of the grinding and polishing section 14. The second region 23 contains a layer that ensures the corrosion resistance of the grinding and polishing section 14, including the welded section 13. As schematically shown in Figure 2, the second region 23 contains a second coating layer 231 formed by a repair treatment using thermal spraying.

[0076] The second region 23 is a region in which intermetallic compounds constituting the intermetallic compound layer 12 do not exist continuously for 20 μm or more.

[0077] The circumferential length of the second region 23 is 4 mm or less. The significance of defining the circumferential length of the second region 23 in this way is not limited to this, but can be explained as follows. As a premise, since the second region 23 is a region formed on the grinding and polishing section 14, its circumferential length is considered to correspond to the width of the grinding and polishing section 14 during the manufacturing process. In other words, if the width of the grinding and polishing section 14 during the manufacturing process is large, the circumferential length of the second region 23 may also exceed 4 mm. If the width of the grinding and polishing section 14 is large, the plating layer on the surface of the steel substrate 11 near the welded section 13 is removed by that width. If the area from which the plating layer has been removed is large, when forming the second coating layer 231 on the surface of the steel substrate 11 by thermal spraying, unevenness in the thermal spraying is likely to occur, and as a result, there may be parts with insufficient thickness. In this case, it may not be possible to sufficiently ensure the corrosion resistance of the steel substrate 11, especially in the thin parts (the parts with the minimum thickness described later). Furthermore, more thermal spray material is required to form a second coating layer 231 of sufficient thickness on the surface of the steel substrate 11 to suppress corrosion.

[0078] In contrast, since the circumferential length of the second region 23 in this embodiment is 4 mm or less, it is ensured that the width of the grinding and polishing portion 14 is kept narrow during the manufacturing of the steel pipe 1. In other words, by keeping the width of the grinding and polishing portion 14 narrow, a second region 23 with a circumferential length of 4 mm or less is formed. As a result, it is possible to ensure that a second coating layer 231 of a more appropriate thickness is formed on the surface of the steel substrate 11 from which the plating layer has been removed in a relatively narrow width.

[0079] Furthermore, as will be described later, a plating layer, or a heat-affected plating layer, remains in the third region 25 adjacent to the second region 23, and together with the second coating layer 231 formed on top of this plating layer, it constitutes a sufficiently thick film layer 20. Therefore, by having a circumferential length of 4 mm or less in the second region 23, it is possible to ensure that a sufficiently thick film layer 20 is formed near the grinding and polishing portion 14, thereby making the corrosion resistance of the steel pipe 1 more favorable. Here, the plating layer of the third region 25 is substantially composed of the same elements as the plating layer of the first region 21. However, depending on the degree of heat input during welding, the plating layer of the third region 25 may be affected by the heat of welding, resulting in a different composition from the plating layer of the first region 21, or oxides may be present.

[0080] Regarding the method for identifying area 23 in the second domain: In any cross-section of the steel pipe 1 according to this embodiment, as shown in Figure 1, obtained by cutting the pipe in the radial direction, the portion corresponding to the second region 23 can be easily identified by observing the cross-section of the steel pipe 1 of interest using SEM-EDX and performing elemental mapping analysis, similar to how the portion corresponding to the first region 21 is identified.

[0081] More specifically, the second region 23 can be identified as follows, focusing on the presence or absence of the Zn-Al-Mg alloy plating layer or the intermetallic compound layer 12. As a premise, the second region 23 in this embodiment is the region corresponding to the grinding and polishing portion 14 of the base steel pipe 10. In the grinding and polishing portion 14, the Zn-Al-Mg alloy plating layer and the intermetallic compound layer 12 are removed by the grinding process. Therefore, by identifying a region near the welded portion 13, near the surface of the steel base 11, that region can be identified as the second region 23 by identifying a region with a different composition from the Zn-Al-Mg alloy plating layer or the intermetallic compound layer 12. This is also true when the second coating layer 231 has a multilayer structure having an Al layer 233 and a Zn layer 235, as described later.

[0082] In one embodiment, first, the portion of the cross section of interest corresponding to the welded portion 13 of the base steel pipe 10 is identified. Next, on the surface of the steel base 11 near the identified welded portion 13, a region within 3 μm in thickness from the surface that does not contain at least one of Zn, Al, and Mg is identified. Since the identified region has a different composition from the Zn-Al-Mg alloy plating layer, it can be distinguished from the Zn-Al-Mg alloy plating layer. In other words, since the identified region does not contain all of the Zn, Al, and Mg that constitute the Zn-Al-Mg alloy plating layer, it is considered to be a portion where the Zn-Al-Mg alloy plating layer was removed during the manufacturing process of the steel pipe 1, i.e., a region on the grinding and polishing portion 14. Therefore, it is possible to identify the region where a second coating layer 231 that does not contain at least one of Zn, Al, and Mg is formed as the second region 23.

[0083] In another embodiment, first, the portion of the cross section of interest corresponding to the welded portion 13 of the base steel pipe 10 is identified. Next, on the surface of the steel base 11 near the identified welded portion 13, a region is identified within a thickness of 3 μm from the surface that contains two of Zn, Al, and Mg, and the remaining one is contained in an amount of 1% or less. This identified region can be distinguished from the Zn-Al-Mg alloy plating layer. In other words, although this identified region contains all of Zn, Al, and Mg that constitute the Zn-Al-Mg alloy plating layer, the content of the remaining one is small, so it is considered to be a portion where the Zn-Al-Mg alloy plating layer was removed during the manufacturing process of the steel pipe 1, i.e., a region on the grinding and polishing portion 14. Therefore, a region where a second coating layer 231 containing two of Zn, Al, and Mg, and the remaining one is contained in an amount of 1% or less, can be identified as the second region. Of the Zn, Al, and Mg that may be contained in the second coating layer 231, two are thought to be components derived from the metal components that were thermally sprayed onto the grinding and polishing section 14. In contrast, the remaining component that may be contained in the second coating layer 231 at a concentration of 1% or less is thought to be a component that has become mixed into the second coating layer 231 through melting or diffusion between it and the Zn-Al-Mg alloy plating layer in the adjacent third region 25.

[0084] In another embodiment, if an intermetallic compound layer 12 is formed on the surface of the steel substrate 11 in the first region 21, the second region 23 can be identified as follows. First, the portion of the cross section of interest corresponding to the welded portion 13 of the steel pipe 10 is identified. Next, on the surface of the steel substrate 11 near the identified welded portion 13, a region is identified in which the intermetallic compound constituting the intermetallic compound layer 12 does not continuously exist for 20 μm or more. This identified region is considered to be the portion where the intermetallic compound layer 12 was removed during the manufacturing process of the steel pipe 1, i.e., the region on the grinding and polishing portion 14. Therefore, a region in which the intermetallic compound constituting the intermetallic compound layer 12 does not continuously exist for 20 μm or more can be identified as the second region 23.

[0085] The circumferential length of the second region 23, as identified above, can be measured in the same manner as the circumferential length of the first coating layer 211. That is, an arbitrary cross section of interest is observed using a SEM at a magnification of approximately 500x, and the length is measured at the interface between the second coating layer 231 and the steel substrate 11 in the second region 23 using the length measuring function implemented in the SEM. In such a measurement, it is sufficient that the circumferential length of the second region 23 is 4 mm or less.

[0086] ≪Regarding the chemical composition of the second coating layer 231≫ In one embodiment, the second coating layer 231 of the second region 23 has a multilayer structure, as schematically shown in Figure 3, which includes an Al layer 233 provided on the surface of the steel substrate 11 in the grinding and polishing section 14 so as to at least cover the welded section 13 of the base steel pipe 10, and a Zn layer 235 on the Al layer 233. In this case, according to one embodiment, the second coating layer 231 has, as described above, an Al layer 233 containing Zn: 0 to 20.0% by mass, with the remainder being Al and impurities, and a Zn layer 235 containing Al: 0.1 to 20.0% by mass, with the remainder being Zn and impurities. The following description will focus on the case in which the second coating layer 231 has a multilayer structure, but the technology of this disclosure is not limited thereto. The second coating layer 231 only needs to contain at least one of Zn and Al. It may contain both Zn and Al, and may also contain one or more of the following elements: Mg, Fe, Ni, Si, and Sn. Furthermore, each element may be layered, in three or more layers, mixed, or the mixed layers may be multilayered. The elements may be alloyed. In addition, during thermal spraying, the metallic elements may oxidize, and some oxides may be mixed in.

[0087] The second coating layer 231 is formed, for example, by thermal spraying. The thermal spraying method is not limited. Gas flame spraying, arc spraying, plasma spraying, or laser spraying may be used. The sprayed metal contains one or more of Zn, Al, and Mg, but if it contains three or more, it is sufficient if one of them is present in an amount of 1% or less. Other elements may also be included, such as elements A to H diffused from the plating layer, or these elements and other elements may be included in the sprayed metal and sprayed to add them to the coating layer. Impurities may also be present. Examples of sprayed metals include Zn, Al, Zn-6%Al, Zn-15%Al, or Al-5%Mg, but the method is not limited to these examples. Furthermore, to further improve corrosion resistance, a chemical conversion treatment may be applied on top of the sprayed layer. The chemical conversion treatment may be organic or inorganic. Painting may also be applied.

[0088] [Method for measuring chemical components] The chemical composition of the second coating layer 231 described above can be determined by performing quantitative analysis using SEM-EDX on the portion of the second coating layer 231 in the second region 23 identified by elemental mapping analysis using SEM-EDX as previously explained. More specifically, for the second coating layer 231 having a multilayer structure according to one embodiment, quantitative analysis using SEM-EDX is performed at arbitrary locations in the portion corresponding to the second coating layer 231, in the Al-based layer located closer to the weld 13, and in the Zn-based layer located above this Al-based layer. Such quantitative analysis can be performed at arbitrary locations in each layer, and the average of the obtained measurement values ​​can be treated as the chemical composition of each layer.

[0089] Regarding the thickness of the second coating layer 231: [Regarding the average thickness of the second coating layer 231] Furthermore, in the steel pipe 1 according to this embodiment, the overall average thickness of the second coating layer 231 is preferably in the range of 10.0 to 150.0 μm, although it is not limited to this range. By having an average thickness of the second coating layer 231 in the range of 10.0 to 150.0 μm, the second coating layer 231 according to this embodiment exhibits better corrosion resistance while suppressing an increase in manufacturing costs. The average thickness of the second coating layer 231 is more preferably in the range of 20.0 to 100.0 μm.

[0090] The average thickness of the second coating layer 231 can be measured by observing an arbitrary cross-section obtained by cutting the steel pipe 1 according to this embodiment in the radial direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1) using a SEM. More specifically, the obtained cross-section is observed at a magnification of approximately 500 times using an SEM, and the length from the interface between the welded part 13 and the Al layer 233 to the surface of the Zn layer 235 is measured at any 10 locations within the observation field using the length measuring function implemented in the SEM. The average thickness of the second coating layer 231 can be calculated by averaging the 10 measurements obtained in this way over the number of measurement locations.

[0091] [Regarding the minimum thickness of the second coating layer 231] In either the case where the second coating layer 231 is composed of multiple layers or a single layer, it is preferable that the minimum thickness of the second coating layer 231 in the second region 23 be 3 μm or more. This makes the corrosion resistance of the steel pipe 1 more favorable.

[0092] The minimum thickness of the second coating layer 231 in the second region 23 can be measured in the same way as the average thickness. That is, the steel pipe 1 according to this embodiment is cut in the diameter direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1), and an arbitrary cross-section is taken. The obtained cross-section is observed using a SEM at a magnification of approximately 500 times, and at any 10 locations in the observation field, the portion of the second coating layer 231 in the second region 23 with the smallest thickness is identified using the length measuring function implemented in the SEM, and its thickness is measured. The smallest of these 10 measured values ​​can then be calculated as the minimum thickness. However, when identifying the portion with the smallest thickness in the above observation, portions with obvious handling defects or processing cracks are excluded. Since handling defects often result in defects in the steel substrate, if there is deformation (indentation) in the adjacent steel substrate, it can be determined to be a handling defect. While it depends on the degree of machining, a machining crack can be identified as such if the crack width directly above the adjacent steel substrate is approximately 10 μm or less.

[0093] Furthermore, the maximum thickness of the second coating layer 231 in the second region 23 is not particularly limited. Naturally, forming a second coating layer 231 with a large maximum thickness requires more thermal spray material. Therefore, from the viewpoint of manufacturing costs, the maximum thickness can be 250 μm or less.

[0094] <Regarding Area 3, Item 25> Next, with reference to Figures 1 and 2, the third region 25 of the coating layer 20 according to this embodiment will be described in detail.

[0095] As previously mentioned, the third region 25 in this embodiment is a region located between the first region 21 and the second region 23, in which the first coating layer 211 undergoes changes such as (a) thinning, (b) slight variation in plating composition, (c) inclusion of oxides, or (d) disappearance. Since the portion corresponding to the first region 21 and the portion corresponding to the second region 23 can be identified by the method described above, the portion corresponding to the third region 25 can be easily identified as a portion located between the first region 21 and the second region 23.

[0096] In one embodiment, the third coating layer 251 of the third region 25 has a configuration in which the second coating layer 231 exists on top of the thinned first coating layer 211, or on top of the steel substrate 11 or intermetallic compound layer 12 that has been exposed due to the disappearance of the first coating layer 211.

[0097] In one embodiment, the third coating layer 251 may include a region formed when the components of the first coating layer 211 and the components constituting the second coating layer 231 mix or react with each other during the repair process. Furthermore, during such a repair process, components of the first coating layer 211 contained in the first region 21 may be mixed in by diffusion.

[0098] Therefore, the chemical composition of the third coating layer 251 according to this embodiment is mainly Zn or Al, and may also contain one or more elements selected from the group consisting of element groups A to H that may be contained in the first coating layer 211 and the second coating layer 231. The elements of element groups A to H are as previously explained.

[0099] [Regarding the average thickness of the third coating layer 251] The average thickness of the third coating layer 251 according to this embodiment is preferably in the range of 8 to 200 μm. By having the average thickness of the third coating layer 251 within the above range, it is possible to further improve the corrosion resistance of the third coating layer 251 according to this embodiment. The average thickness of the third coating layer 251 is more preferably 15 μm or more. Furthermore, the average thickness of the third coating layer 251 is more preferably 18 μm or less.

[0100] Here, the average thickness of the third coating layer 251 can be measured by cross-sectional observation using SEM as follows. More specifically, we focus on an arbitrary cross-section obtained by cutting the steel pipe 1 according to this embodiment in the radial direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1). The obtained cross-section is observed at a magnification of approximately 500 times using a SEM, and the thickness of the third coating layer 251 is measured at 10 arbitrary locations within the observation field using the length measuring function implemented in the SEM. The average thickness of the third coating layer 251 can be calculated by averaging the 10 measurements obtained in this way over the number of measurement locations.

[0101] In one embodiment, the third region 25 includes a partial region 25a extending 1 mm in the circumferential direction from the end of the second region 23 in the opposite direction to the second region 23 (the direction of the third region 25). The partial region 25a corresponds to region B of the present disclosure. The partial region 25a included in the third region 25 includes a first coating layer 211 which is a Zn-Al-Mg alloy plating layer. The partial region 25a of the third region 25 also includes a second coating layer 231 formed on the first coating layer 211. In other words, the third region 25 has a first coating layer 211 which is a Zn-Al-Mg alloy plating layer in the partial region 25a, and includes a second coating layer 231 which is a metal layer containing one or more metals, either Zn or Al, on the first coating layer 211.

[0102] In one embodiment, the average thickness of the first coating layer 211 in partial region 25a is 50% or more of the average thickness of the first coating layer 211 in a partial region within a circumferential range of 1 mm at an arbitrary position in the first region 21. The partial region within a circumferential range of 1 mm at an arbitrary position in the first region 21 is a position in the first region 21 that is not affected by the heat input during welding, and can be located 10 to 15 mm from the edge of the second region 23. This partial region of the first region 21 corresponds to region C of the present disclosure.

[0103] [Regarding the minimum thickness of the third coating layer 251] The minimum thickness of the third coating layer 251 in the third region 25 is preferably 3 μm or more, at least in the partial region 25a described above. This makes the corrosion resistance of the steel pipe 1 more favorable.

[0104] The minimum thickness of the third coating layer 251 can be measured in the same way as the average thickness. That is, the steel pipe 1 according to this embodiment is cut in the diameter direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1), and an arbitrary cross-section is taken. The obtained cross-section is observed using a SEM at a magnification of approximately 500 times, and the portion of the third coating layer 251 with the smallest thickness is identified at any 10 locations within the observation field of view using the length measuring function implemented in the SEM, and its thickness is measured. The smallest of these 10 measured values ​​can then be calculated as the minimum thickness.

[0105] The steel pipe 1 according to this embodiment has been described in detail above with reference to Figures 1 to 3.

[0106] In the above description of the embodiment, the coating layer 20 on the steel pipe 1 was described by dividing it into a first region 21, a second region 23, and a third region 25. However, it is also possible to describe other features of the coating layer 20 in relation to the steel pipe 1 according to this disclosure. Figure 4 is a schematic enlarged cross-sectional view showing a cross-section near the weld in a steel pipe 1 according to one embodiment, and is an explanatory diagram for explaining other features of the steel pipe 1.

[0107] In one embodiment of the steel pipe 1, as shown in Figure 4, a fourth region 27 is defined as the entire region formed by the repair treatment of the grinding and polishing portion 14 in the coating layer 20, independently of the first region 21, second region 23, and third region 25 described above. The fourth region 27 is a region on the surface of the steel substrate 11 in the range from the center of the welded portion 13 of the raw steel pipe 10 (center line L in the figure) to both sides in the circumferential direction, up to 5 mm on each side.

[0108] When using this definition, the minimum thickness of the fourth region 27 of the coating layer 20 is 3 μm or more. By defining the minimum thickness of the fourth region 27 in this way, it is possible to ensure that a coating layer 20 of sufficient thickness is formed in the vicinity of the grinding and polishing portion 14, thereby making the corrosion resistance of the steel pipe 1 more favorable.

[0109] The minimum thickness of the fourth region 27 in the coating layer 20 can be measured in the same way as the minimum thickness of the second coating layer 231 or the third coating layer 251 described above. That is, the steel pipe 1 according to this embodiment is cut in the diameter direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1), and an arbitrary cross-section is taken. The obtained cross-section is observed at a magnification of approximately 500 times using a SEM, and the portion of the coating layer 20 with the smallest thickness is identified at any 10 locations in the observation field using the length measuring function implemented in the SEM, and its thickness is measured. The smallest of these 10 measured values ​​can then be calculated as the minimum thickness.

[0110] (Regarding the manufacturing method of steel pipes) Next, an example of a manufacturing method for the steel pipe 1 according to this embodiment will be briefly described below.

[0111] <Manufacturing method for Zn-Al-Mg alloy plated steel sheet, which is the raw material> The Zn-Al-Mg alloy plated steel sheet used as the material for the steel pipe 1 according to this embodiment is manufactured by using the steel material described above as the base material and forming a Zn-Al-Mg alloy plating layer on the surface of the steel material.

[0112] In this context, the Zn-Al-Mg alloy plating layer can be formed using methods such as hot-dip galvanizing, thermal spraying, cold spraying, sputtering, vapor deposition, and electroplating. However, hot-dip galvanizing is the most preferable method in terms of cost.

[0113] The following describes in detail an example of a manufacturing method for obtaining a Zn-Al-Mg alloy plating layer using a hot-dip galvanizing method. In the manufacturing process of such a Zn-Al-Mg alloy plating layer, first, a steel sheet, as an example of the steel material to be used as the base material, is rolled to the desired thickness using the Zenzimir method, then wound into a coil, and placed on the hot-dip galvanizing line.

[0114] In a hot-dip galvanizing line, steel sheets are continuously fed through a coil. At this time, the steel sheets are heated and reduced at 800°C in an annealing facility installed on the line, for example, in an environment where oxidation is unlikely to occur, such as an oxygen concentration of 20 ppm or less, in an N2-5%H2 gas atmosphere. After that, they are air-cooled with N2 gas to approximately 20°C above the temperature of the subsequent plating bath, and then immersed in the plating bath.

[0115] Here, a molten plating alloy having the chemical composition of the first coating layer 211 as described above is prepared in the plating bath. The temperature of the plating bath is set to be above the melting point of the plating alloy (for example, around 460-660°C).

[0116] When preparing the material for the plating alloy, it is preferable to use pure metal (purity of 99% or higher) as the alloying material. First, a predetermined amount of alloying metal is mixed to obtain the composition of the plating layer as described above, and then completely melted into an alloy using a high-frequency induction furnace or arc furnace under vacuum or inert gas displacement conditions. Furthermore, the alloy mixed with the predetermined components (composition of the plating layer described above) is melted in the atmosphere, and the resulting molten material is used as the plating bath.

[0117] Furthermore, there are no particular restrictions on using pure metals when producing the plating alloys described above; existing Zn alloys, Mg alloys, and Al alloys can be melted and used. In this case, as long as an alloy with a predetermined composition and low impurity levels is used, there will be no problems.

[0118] After immersing the steel plate in the plating bath described above, it is withdrawn at a predetermined speed. At this time, the amount of plating deposited is controlled, for example, by N2 wiping gas, so that the zinc-based plating layer formed reaches the desired thickness. Here, conditions other than the bath temperature can be those of general plating operation, and no special equipment or conditions are required.

[0119] Furthermore, various heat treatments may be applied to the molten plated alloy located on the steel plate, as needed. Chemical conversion treatment may also be applied after plating. The chemical conversion treatment may be organic or inorganic, and painting may also be performed.

[0120] <Manufacturing of welded galvanized steel pipes> The Zn-Al-Mg alloy plated steel sheet manufactured as described above is subjected to bending to bring the widthwise ends of the plated steel sheet together, and the joined ends are welded. Here, the conditions for bending the plated steel sheet and the welding conditions are not particularly specified, and various known conditions and methods can be used as appropriate. As a result, a welded plated steel pipe can be obtained in which the Zn-Al-Mg alloy plated layer (i.e., the first coating layer 211) is located on the surface of the steel base 11 in the base steel pipe 10 composed of the steel base 11 and the welded part 13.

[0121] <Grinding process for protruding parts formed by welding> The welded and plated steel pipes manufactured as described above have protruding portions formed by the welding process, as explained earlier. Therefore, these protruding portions are ground down. At this time, the circumferential length of the ground and polished portion 14 created by the grinding is adjusted to be 4 mm or less. This makes it possible to make the circumferential length of the second region 23 of the coating layer 20 formed in the subsequent process 4 mm or less.

[0122] In conventional manufacturing processes, polishing is sometimes performed around the protruding portion before the grinding process described above to prevent the generation of processing debris from the plating layer. However, this polishing process can remove the plating layer over a wide area near the protruding portion. In this case, it is not possible to obtain the second region 23 with a circumferential length of 4 mm or less, as is the case with the coating layer 20 of the steel pipe 1 according to this embodiment. Therefore, in the manufacturing method of the steel pipe 1 according to this embodiment, it is important not to perform a polishing process that removes the plating layer near the protruding portion before removing the protruding portion of the welded portion 13 by grinding. Alternatively, even if polishing is performed, it is important to perform polishing to the extent that the plating layer is not removed. This allows the plating layer to remain near the grinding and polishing portion 14. This makes it possible to set the circumferential length of the second region 23 to the desired value. Furthermore, it is possible to set the minimum thickness of the third region 25 to the desired value. Therefore, the corrosion resistance of the steel pipe 1 can be made satisfactory.

[0123] <Repair treatment of welded joints> In order to ensure the corrosion resistance of the welded portion whose protruding part has been ground down as described above, a repair treatment as described below is performed to form a second coating layer 231 that at least covers the welded portion.

[0124] [Fume removal treatment] Here, generally, prior to the formation of the second coating layer 231, fumes that may be present in the ground portion of the protruding part and its surrounding area are removed. Various methods exist for removing such fumes, including physical and chemical methods, but physical removal methods (mechanical removal methods) are often employed. Examples of physical removal methods (mechanical removal methods) for fumes include grinding using a grinder, grinding using a wire brush, and blasting.

[0125] When the fume removal process described above is performed, the fumes are removed, but along with the fumes, a portion of the first coating layer 211 that was present near the grinding and polishing section 14 is also removed.

[0126] Therefore, in the manufacturing method of the steel pipe 1 according to this embodiment, it is important not to perform a fume removal treatment that removes the plating layer after removing the protruding portion of the welded portion 13 by grinding. Alternatively, even if a fume removal treatment is performed, it is important to perform the fume removal treatment to an extent that does not remove the plating layer. This makes it possible to leave the plating layer near the ground and polished portion 14, and to set the circumferential length of the second region 23 to the desired value. In addition, the minimum thickness of the third region 25 can be set to the desired value. Thus, the corrosion resistance of the steel pipe 1 can be made satisfactory.

[0127] [Thermal spraying treatment] As described above, the ground and polished portion 14 from which the protruding portion has been removed by grinding is subjected to thermal spraying. The thermal spraying method for the thermal spraying material is not particularly limited, and known thermal spraying methods such as arc spraying and gas flame spraying can be used. Furthermore, there are no specific thermal spraying conditions that are prescribed. For the thermal spray material used in the thermal spraying process, in order to achieve the desired chemical composition of the second coating layer 231, a metal element or alloy containing the desired element may be used as appropriate.

[0128] ≪Thermal spray temperature≫ In the manufacturing method of the steel pipe 1 according to this embodiment, the thermal spray temperature is not particularly limited, and any desired known conditions can be applied. Prior to carrying out the thermal spray treatment as described above, it is preferable to control the steel pipe temperature to, for example, within the range of 100.0 to 900.0°C. This makes it easier for the second coating layer 231 to adhere to the surface of the steel substrate 11 or onto the first coating layer 211.

[0129] ≪Cooling rate of steel pipes to room temperature after thermal spraying≫ Furthermore, there are no specific requirements regarding the cooling rate and cooling method after thermal spraying; various known cooling methods such as air cooling and water cooling can be employed.

[0130] The above briefly describes an example of a method for manufacturing the steel pipe 1 according to this embodiment. [Examples]

[0131] The steel pipes according to the present invention will be described in more detail below, with reference to examples and comparative examples. Note that the following examples are merely examples of steel pipes according to the present invention, and the steel pipes according to the present invention are not limited to the examples below.

[0132] (Example test) A 2.3 mm thick general structural rolled steel sheet (SS400) was plated using a hot-dip galvanizing process with a hot-dip galvanizing bath having the composition shown in Table 1. During this process, by appropriately controlling conditions such as the gas blowing pressure during gas wiping, a single-sided plating amount of 120 g / m² was achieved. 2 The composition was adjusted accordingly. The plating layer of the fabricated plated steel sheet was analyzed by ICP and confirmed to have almost the same composition as the plating bath.

[0133] Example 21 involves electroplating 1 g / m² of nickel onto the steel material before plating. 2 The process was carried out before plating. Example 22 involved electroplating of 1 g / m² of copper onto the steel material before plating. 2 The plating was performed after the procedure. In Example 23, the steel material was electroplated with 1 g / m² of zinc before plating. 2 The plating was done after the procedure.

[0134] The prepared plated steel sheets were formed into open pipes with a diameter of 139.8 mmφ, butt-jointed, and plated steel pipes were fabricated by high-frequency welding. After cutting the welded bead sections to various widths, arc spraying was performed using the thermal spray wires shown in Table 3.

[0135] A portion of the steel pipe after thermal spraying was cut radially, perpendicular to the longitudinal direction, and the cross-section was observed using SEM-EDX. Region A was identified as the area where Fe-Al intermetallic compounds were not continuously present for 20 μm or more within 3 μm thickness from the steel substrate. The circumferential length of Region A was also measured. The elements contained in the metal layer of Region A were confirmed using an EDX analyzer. The minimum and average thickness of the metal layer in Region A was also measured. Furthermore, a 1 mm circumferential region was identified from both ends of Region A in the opposite direction to Region A. Region B was identified. In Region B, the plating layer formed on the original plated steel sheet was identified, and the minimum thickness of this plating layer was measured. Furthermore, it was confirmed whether a metal layer existed on top of this plating layer in Region B. The elements contained in the metal layer of Region B were confirmed using an EDX analyzer. Furthermore, a 1 mm circumferential region located 15 mm away from the ends of Region A in the opposite direction to Region A was identified as Region C. Furthermore, the average thickness of the plating layer in region C was measured. The ratio of the average thickness of the plating layer in region B to the average thickness of the plating layer in region C (the "Average Thickness Ratio of Plating Layer in Region B" [%] in Table 2) was also calculated. In addition, the minimum thickness was measured in a region extending 5 mm on each side of the circumferential direction from the center of the weld. These results are shown in Table 2. Note that, regarding the area where region A exists, the part other than the weld is described as the general area.

[0136] The remaining steel pipe not used for the above cross-sectional observation was cut in half lengthwise so that the thermal sprayed area was at the center, dividing it into a sample with thermal spray (Y sample) and a sample without thermal spray (M sample), and the cut ends of each were sealed. Each sealed sample was subjected to a combined cycle accelerated corrosion test (CCT) as specified in JIS H8502:1999, and the test was carried out for a maximum of 600 cycles, with a corrosion resistance evaluation of 1. The number of cycles y at which the area of ​​red rust occurrence rate of the Y sample becomes 10% was judged as ◎ if it was the same as or greater than the number of cycles m at which the area of ​​red rust occurrence rate of the M sample becomes 10% (y≧m), ○ if it was 0.9 times (y / m=0.9), and × if it was less than 0.9 times (y / m<0.9). In addition, if the area of ​​red rust occurrence rate of the Y sample was less than 10% even after 600 cycles, it was judged as ◎, and if the area of ​​red rust occurrence rate of either the Y sample or the M sample was 10% or more after 150 cycles, it was judged as ×. ◎ and ○ were selected as passing examples, while × was selected as failing examples. Furthermore, the CCT test was extended to 900 cycles, and corrosion resistance evaluation 2 was performed. In this test, if the number of cycles y2 at which the area of ​​red rust occurrence rate of the Y sample reached 10% was the same as or greater than the number of cycles m2 at which the area of ​​red rust occurrence rate of the M sample reached 10% (y2≧m2), it was marked as ◎; if it was 0.9 times (y2 / m2=0.9), it was marked as ○; and if it was less than 0.9 times (y2 / m2<0.9), or if the area of ​​red rust occurrence rate of M exceeded 50% at 720 cycles, it was marked as ×. These results are shown in Table 2.

[0137] Comparative Examples 29 and 30 had low corrosion resistance due to the plating itself, resulting in a red rust occurrence rate of 10% or more in less than 150 cycles. Comparative Example 27 had poor corrosion resistance because the plating in area B was removed due to polishing treatment before bead cutting.

[0138] In Example 18-2, an organic chemical conversion treatment was applied after plating before pipe manufacturing, and in Example 18-3, an inorganic chemical conversion treatment was applied after plating before pipe manufacturing. Both resulted in excellent corrosion resistance (◎). In Example 18-3, an organic chemical conversion coating was applied directly above the thermal sprayed area after pipe manufacturing, and also resulted in excellent corrosion resistance (◎).

[0139] [Table 1]

[0140] [Table 2]

[0141] [Table 3]

[0142] As is clear from Table 2 above, the steel pipe corresponding to the embodiment of the present invention exhibited excellent corrosion resistance, while the steel pipe corresponding to the comparative example of the present invention had insufficient corrosion resistance.

[0143] Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these are also understood to fall within the technical scope of the present invention.

[0144] The embodiments disclosed herein are illustrative and not restrictive in all respects. The embodiments described above may be omitted, replaced, or modified in various ways without departing from the appended claims, the technical scope of the invention as described later, and the spirit thereof. For example, the constituent elements of the embodiments described above can be combined in any way without impairing their effects. Furthermore, such any combination will naturally yield the effects and benefits of each constituent element in the combination, as well as other effects and benefits that will be obvious to those skilled in the art from the description herein.

[0145] Furthermore, the effects described herein are merely descriptive or illustrative, and not limiting. In other words, the technology according to the present invention may produce other effects that will be apparent to those skilled in the art from the description herein, in addition to or instead of the effects described above. [Explanation of symbols]

[0146] 1 Steel pipe 10 Untreated steel pipes 11 Steel base 13. Welded section 20 Coating layer 21 First area 211 1st coating layer 23 Second area 231 Second coating layer 233 Al layer 235 Zn layer 25 Third area 251 Third coating layer

Claims

1. It is a plated steel pipe, At least on the outer surface of the plated steel pipe, a coating layer containing a Zn-Al-Mg alloy plating layer exists on the steel substrate. In any cross-section obtained by cutting the aforementioned plated steel pipe in the radial direction perpendicular to the longitudinal direction, The aforementioned coating layer is The surface of the steel substrate has a region A where Fe-Al intermetallic compounds are not continuously present for 20 μm or more. The circumferential length of region A is 4 mm or less. In the region A, there is a metal layer containing one or more metals, namely Zn and Al. The minimum thickness of the coating layer in region A is 3 μm or more. Galvanized steel pipe.

2. The aforementioned coating layer is Region A has a region B that extends 1 mm in the circumferential direction from the end of region A in the opposite direction to region A, The plated steel pipe according to claim 1, wherein the region B has the Zn-Al-Mg alloy plating layer, and the Zn-Al-Mg alloy plating layer has a metal layer containing one or more metals, either Zn or Al.

3. The aforementioned coating layer is Region C is a region that is 1 mm in the circumferential direction at any position within a range of 10 to 15 mm in the circumferential direction from the end of region A in the opposite direction to region A. The average thickness of the Zn-Al-Mg alloy plating layer in region B is 50% or more of the average thickness of the coating layer in region C. The plated steel pipe according to claim 2, wherein the minimum thickness of the coating layer in region B is 3 μm or more.

4. The plated steel pipe according to any one of claims 1 to 3, wherein the region A is a region that covers the welded portion of the base steel pipe constituting the plated steel pipe.

5. The Zn-Al-Mg alloy plating layer is, by mass%, Al: more than 1.0% but not more than 30.0%, Mg: More than 1.0% and 15.0% or less, The plated steel pipe according to any one of claims 1 to 3, having a chemical composition consisting of the remainder being Zn and impurities.

6. The Zn-Al-Mg alloy plating layer is, by mass%, Al: more than 1.0% but not more than 30.0%, Mg: more than 1.0% and less than 15.0% A plated steel pipe according to claim 1 or 2, which contains, and further contains one or more elements selected from the group consisting of element groups A to E below, with the remainder being Zn and impurities, having a chemical composition. [Element group A]: Fe: 5.00% or less [Element Group B]: One or more elements selected from the group consisting of Ti: 0.25% or less, Ni: 1.00% or less, and Co: 0.25% or less. [Element group C]: Sn: 0.70% or less [Element group D]: Ca: 0.60% or less [Element group E]: B: 0.50% or less

7. The Zn-Al-Mg alloy plating layer is, by mass%, Al: more than 1.0% but not more than 30.0%, Mg: more than 1.0% and less than 15.0% The plated steel pipe according to claim 3, further containing one or more elements selected from the group consisting of element groups A to E below, with the remainder being Zn and impurities, having a chemical composition. [Element group A]: Fe: 5.00% or less [Element Group B]: One or more elements selected from the group consisting of Ti: 0.25% or less, Ni: 1.00% or less, and Co: 0.25% or less. [Element group C]: Sn: 0.70% or less [Element group D]: Ca: 0.60% or less [Element group E]: B: 0.50% or less

8. The plated steel pipe according to claim 7, wherein the Zn-Al-Mg alloy plating layer having at least one of region B and region C has a chemical composition having the element group A.

9. The plated steel pipe according to claim 7, wherein the Zn-Al-Mg alloy plating layer having at least one of region B and region C has a chemical composition having the element group B.

10. The plated steel pipe according to claim 7, wherein the Zn-Al-Mg alloy plating layer having at least one of region B and region C has a chemical composition having the element group C.

11. The plated steel pipe according to claim 7, wherein the Zn-Al-Mg alloy plating layer having at least one of the region B and the region C has a chemical composition having the element group D.

12. The plated steel pipe according to claim 7, wherein the Zn-Al-Mg alloy plating layer having at least one of the region B and the region C has a chemical composition having the element group E.

13. It is a plated steel pipe, At least on the outer surface of the plated steel pipe, a coating layer containing a Zn-Al-Mg alloy plating layer exists on the steel substrate. In any cross-section obtained by cutting the aforementioned plated steel pipe in the radial direction perpendicular to the longitudinal direction, The coating layer has a minimum thickness of 3 μm or more in a region extending 5 mm on each side of the circumferential direction from the center of the welded area. Galvanized steel pipe.

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